紊流激励条件下基于随机减量法的颤振边界预测研究

发展了一种紊流激励条件下的颤振边界预测方法。首先基于随机减量法从紊流激励响应中提取出系统的自由衰减响应,其次采用矩阵束法从自由衰减响应中识别出振动响应的主要模态参数,最后用识别出来的模态参数构造稳定性判据,根据稳定性判据与动压或风速的变化关系用曲线外推来预测颤振边界。数值仿真与风洞试验研究表明:利用随机减量法和矩阵束法能够较准确地识别出紊流激励响应中的主要模态参数;稳定性颤振判据下降趋势明显、规律性好,有助于在较低风速下提前预测出颤振边界,从而减小风洞试验或颤振试飞的安全隐患。     

民用飞机气动伺服弹性试飞激励响应仿真研究

为提高民用飞机气动伺服弹性试飞仿真预测能力,实现高效安全的民用飞机气动伺服弹性试飞。在民用飞机气动伺服弹性试飞方法的基础上,提出民用飞机气动伺服弹性试飞激励响应仿真方法。该方法以民用飞机全机动力学有限元模型为基础,建立带飞行控制律的飞机气动伺服弹性模型,通过副翼、升降舵和方向舵的激励分别实现对飞机的激励响应仿真,得到飞机结构响应量值。为进一步验证该方法的可行性,进行某民用飞机副翼脉冲激励响应仿真,并将仿真响应结果与试飞结果对比,响应幅值相差15.3%,满足工程要求。民用飞机气动伺服弹性试飞仿真很好地预测试飞激励的飞机响应,为试飞激励信号的优化以及结构响应的评估提供技术参考。     

紊流激励下基于贝叶斯统计推断的颤振边界预测方法研究

提出将贝叶斯统计推断方法推广应用于大气紊流激励下飞行器结构的颤振分析，对含不确定性因素影响的模态参数识别与颤振边界预测进行研究。在采用自然激励技术从结构在大气紊流激励下的响应中提取自由衰减信号后，基于贝叶斯统计推断，通过马尔科夫链蒙特卡罗(Markov chain Monte Carlo, MCMC)算法对结构模态参数的后验概率密度函数进行采样识别，并利用Z-W(Zimmerman-Weissenburger)颤振裕度法获取颤振速度概率分布，预测颤振边界并分析其不确定性。进行了数值仿真研究，对大气紊流激励下的结构响应数据进行分析，验证了所提出方法的有效性。     

飞机颤振试飞试验信号的广义时频滤波

针对飞机颤振试飞试验信号噪声过大的问题,提出了一种广义时频域滤波算法。算法采用分数阶傅里叶变换对线性扫频激励及其响应信号进行广义时频分析,利用该类信号在分数阶傅里叶域内的聚焦特性,有效提取真实响应信号,达到信噪分离的目的。给出了具体的滤波算法,并将其应用于仿真算例和实际试飞数据,结果表明该方法可显著提高信号的信噪比。     

时频域滤波及在飞机颤振试飞试验中的应用

针对飞机颤振试飞试验信号噪声过大的问题，提出了一种时频域滤波算法。采用Modet小波对扫频激励及其响应进行时频分析，利用该类信号聚焦于时频域特定区域的特性，有效地提取真实响应信号，达到信噪分离的目的。给出了具体的滤波算法，并研究了信号的重构问题。最后将上述方法应用于仿真算例和实际试飞数据，结果表明该方法显著的提高了频响函数的估计精度。     

飞机紊流激励响应的模态参数识别

风洞颤振试验中通常采用紊流自然激励,提高了模态参数识别的难度。发展了两类紊流激励响应的模态参数识别方法:一是采用随机子空间方法结合经验模态分解方法识别信号的模态参数;二是采用随机减量技术、自然激励技术提取自由衰减响应,进而利用矩阵束方法进行参数识别。紊流激励响应仿真数据与风洞试验数据分析结果表明:采用这两类方法都可以进行单测点紊流激励响应信号的模态参数识别,识别结果具有较高的准确性,结合速度-阻尼比法与颤振裕度法,有助于颤振边界的提前预测。     

某型飞机颤振试飞中多种激励技术的综合应用与研究

文章首先对颤振试飞所用激励技术进行了简单介绍。进而对某型飞机颤振试飞所选激励技术的原理进行了详细分析,并通过对该型飞机颤振试飞不同激励技术激励效果的分析讨论,给出针对该型飞机,多种激励技术补充使用所得激励效果明显的结论。最后对其它飞机颤振试飞激励方法的选用给出有益建议。     

桥梁颤振气动导数识别的ERA算法及其仿真实现

颤振气动导数识别是桥梁颤振理论研究的核心与关键问题,现有文献的主要方法是基于对系统自由振动响应数据的拟合,通过时域模态分析方法识别颤振气动导数.针对均匀流场中桥梁气动弹性的非经典线性动力系统,运用基于相关函数矩阵的特征系统实现法,通过分析系统在不可测量的白噪声激励下的响应,辨识出桥梁非经典模型的特征参数,并利用这些参数恢复模型的系统矩阵进而实现最终的颤振气动导数的识别.数值仿真算例表明本文的方法具有较高的数值稳定性和辨识精度.     

π 型叠合梁颤振试验及数值研究

提出了一种基于改进变分模态分解(Variational Mode Decomposition,VMD)的模态参数辨识算法,用于颤振试验信号的数据处理。采用自然激励技术提取脉冲响应信号;利用信号的先验信息结合本文提出的适应度函数,求解最优分解参数;用参数优化后的VMD算法将信号分解为指定个数的信号分量,每个分量仅含单一频率的振动模态;用矩阵束法识别模态参数。数值仿真和风洞试验研究表明:改进的VMD算法可以有效分离颤振试验信号中的密集模态,提高模态参数辨识的精度;结合颤振裕度法,有助于颤振边界的预测。     

体自由度颤振飞行试验的边界预测方法研究EI北大核心CSCD

体自由度颤振频率低,参与颤振的模态频率在亚临界状态往往已经极为接近,加之基于大气紊流激励的颤振飞行试验数据信噪比通常较低,增加了体自由度颤振飞行试验模态辨识以及颤振预测的难度。对此提出了一种基于Matrix Pencil模态辨识方法的体自由度颤振预测方法。通过随机减量技术对输出响应信号进行系集平均,得到随机衰减标记;运用Matrix Pencil方法拟合随机衰减标记获取模态参数,并通过频率和阻尼稳定判据筛选真实模态,再通过阻尼比与颤振稳定性判据变量外插获取颤振点。通过对仿真数据与试验数据的应用,可得到以下结论:Matrix Pencil模态辨识方法能有效辨识密集的颤振模态,并获得清晰的模态辨识稳态图。基于阻尼比及稳定性判据变量外插获得的颤振预测结果较为合理,其中DTFM(Discrete-Time Flutter Margin)判据变量的下降趋势更明显,外插结果与试验值更接近。该方法适用于体自由度颤振飞行试验的亚临界预测。     

Insertion of triangulated surfaces into a meccano tetrahedral discretization by means of mesh refinement and optimization procedures

In this paper, we present a new method for inserting several triangulated surfaces into an existing tetrahedral mesh generated by the meccano method. The result is a conformal mesh where each inserted surface is approximated by a set of faces of the final tetrahedral mesh. First, the tetrahedral mesh is refined around the inserted surfaces to capture their geometric features. Second, each immersed surface is approximated by a set of faces from the tetrahedral mesh. Third, following a novel approach, the nodes of the approximated surfaces are mapped to the corresponding immersed surface. Fourth, we untangle and smooth the mesh by optimizing a regularized shape distortion measure for tetrahedral elements in which we move all the nodes of the mesh, restricting the movement of the edge and surface nodes along the corresponding entity they belong to. The refining process allows approximating the immersed surface for any initial meccano tetrahedral mesh. Moreover, the proposed projection method avoids computational expensive geometric projections. Finally, the applied simultaneous untangling and smoothing process delivers a high‐quality mesh and ensures that the immersed surfaces are interpolated. Several examples are presented to assess the properties of the proposed method.     

The Research of Anti-Wear Properties of Organomolybdenum Compounds in the Engine Oils

A four-ball tester was used to evaluate the anti-wear performance of three kinds of organomolybdemun compounds in the engine oils, i. e., molybdenum dialkyldithiophosphate (MoDDP), molybdenum dialkyldithiocarbamate ( MoDTC), and sulphur and phosphorus freeorganomolybdeum (Molybdate). The results indicate that a low concentration of MoDDP doesn' t improve the anti-wear properties of the commercial engine oils, but a high concentration of MoDDP can obviously improve the anti-wear properties and the load-carrying capacity of the engine oils. MoDTC doesn' t improve the antiwear properties of the engine oils, but worsens the anti-wear properties of the oils. Signifi can timprove ment of frictional and wear characteristics is obtained with Molybdate added in the commercial engine oils and the formulated oils.     

Certification Institutions Expected to Update and Transfer Information on Present Standards

Standards are the basis for production enterprises to organize production, ex-factory inspection, trade (delivery) and technical exchanges, product certification, quality arbitration and supervision.……     

Flow calorimeter for enthalpy increment measurements on condensed gases

A flow calorimeter for enthalpy increment measurements on condensed gases is presented. A better knowledge of the properties of the liquefied natural gas is needed, and therefore a liquid loop has been designed for our flow calorimeter. The fluid loop in the calorimeter is designed in order to avoid the two-phase region, since two phases would give compositional disturbances in the measurements. The avoidance of the two-phase region is made possible by increasing the pressure of the test fluid after the measurement section, then heating the fluid at super-critical pressure past the critical point. Finally, the fluid is throttled to the low-pressure gas state at the inlet condition of the compressor that circulates the fluid. To perform the pressure increase, a new cryogenic pump has been designed. To evaluate the new equipment, measurements were taken on liquid ethane over the temperature range 146–256 K at pressure between 0.9 and 5.1 MPa.     

Certification Becomes Important for the I Quality; and Safety Supervision of the Chinese Liquor Sector

The end of 2007, over 200 unit products of more than 80 Chinese firms have passed the quality grade certification for liquor products. These products involve distilled spirits, beer, wine, yellow wine, fruit syrup wine and others, and cover over 80% of the national top-branded liquor products.……     

Deposition and Coating Properties on CVD Tungsten

Surface characterization and microstructure studies are performed on chemical vapor deposited (CVD) tungsten coating. There is about 2 μm thickness diffusion layer of tungsten in the molybdenum substrate. The thermal shock test shows tungsten coating has good adhesion with molybdenum substrate, but the elements of oxygen and carbon in the tungsten coating have the bad affection to the adhesion. The result of high-temperature diffusion experiment is the diffusion rate from molybdenum substrate to tungsten coating is faster.     

Refrigerators Enter a New Era of Health Preservation Frestech initiates the establishment of China's first standard for household sterilization refrigerators

On November 30, 2007, the China Association for Standardization (CAS) held a press conference at Beijing Diaoyutai State Guest House. Leaders from the China Household Electric Appliance Research Institute, the China Household Electric Appliance Association, and the China Consumers' Association attended and made speeches.……     

Analysis of marine hydrobiont lipid extracts as possible cryoprotective agents

We studied a combined effect of the cryoprotectants both of lipid and carbohydrate origin at the presence of DMSO or without it on cell viability and the RNA synthesis in the embryonic mollusc and echinoderm cell cultures obtained at different developmental stages. Cryoprotective properties of exogenous lipids correlated with their thermotropic behavior. Lipid extracts from marine hydrobiontes, which thermal transitions are characterized by relatively small part of calorimetric thermogram area lying at temperatures above 0 °C (6–18%), were found to possess a considerable cryoprotective activity. The results are discussed in relation to the prediction of the cryoprotective activity of lipid extracts for marine animal cells. In addition, it was found that marine invertebrate cells can be cryopreserved without DMSO at the presence of trehalose and lipid extracts of some marine hydrobiontes.     

Low-Temperature Properties of Silver

Pure silver is used extensively in the preparation of high-temperature superconductor wires, tapes, films, and other configurations in which the silver not only shields the superconducting material from the surrounding materials, but also provides a degree of flexibility and strain relief, as well as stabilization and low-resistance electrical contact. Silver is relatively expensive, but at this stage of superconductor development, its unique combination of properties seems to offer the only reasonable means of achieving usable lengths of conductor. In this role, the low-temperature physical (electrical, thermal, magnetic, optical) and mechanical properties of the silver all become important. Here we present a collection of properties data extracted from the cryogenic literature and, to the extent possible, selected for reliability.